Catalytic converters have been widely utilized with internal combustion engines, such as used in automobiles, to convert noxious exhaust gas components into harmless gases, with the intention being particularly directed to promoting the emission of carbon monoxide, hydrocarbons and oxides of nitrogen into carbon dioxide, water and nitrogen. The principal components of a typical catalytic converter are a housing having exhaust gas inlet and outlet ports and a catalytic core element enclosed within the housing. The catalytic core element of a conventional catalytic converter typically utilize a multichannel or honeycomb ceramic substrate having a catalyst deposited thereon; or catalyst coated refractory metal oxide beads or pellets; or a corrugated thin metal foil honeycomb monolith having a catalyst carried on or supported on its surface.
The catalyst typically used for catalytic converters is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture or combination of two or more of such metals. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant materials are converted to harmless by-products which then pass through the exhaust system into the atmosphere.
One problem associated with catalytic converters having catalytic core elements utilizing multichannel or honeycomb ceramic substrates is the rigid nature of the ceramic substrates which make them prone to cracking when subjected to thermal stresses and vibrations, such as, for example, those encountered when used in catalytic converters for automobiles or other vehicles. In order to reduce the risk of fracture, the ceramic substrates are often surrounded with a flexible material shrouding. The use of such shrouding, however, substantially increases the difficulty and cost of manufacture. Further, in many applications, such as for use in catalytic converters for use in automobiles and small appliances and equipment, severe space limitations are often encountered which restrict the size and shape of the catalytic converter. Unfortunately, however, the cross-sectional shapes of the ceramic multichannel or honeycomb substrates, which can be economically manufactured for use as catalytic converter core elements, are round or oval in configuration. Thus, the use of such substrates is often undesirable. Another problem associated with such catalytic converters having catalytic core elements utilizing multichannel or honeycomb ceramic substrates is the time consuming and relatively expensive production methods necessary to install such fragile catalytic core elements in the catalytic converter housing.
One problem associated with catalytic converters having catalytic core elements which utilize catalyst coated refractory metal oxide beads or pellets is bead or pellet fluidization. A catalytic converter must survive the turbulent hot exhaust stream and complete the combustion of the gases. During operation, the hot exhaust flow can agitate, swirl and grind the beads or pellets until the function of the converter significantly deteriorates. Accordingly, catalytic converters having such catalytic core elements utilizing catalyst coated refractory metal oxide beads or pellets often require repair or replacement which significantly increases the cost of their use.
One problem associated with catalytic converters having catalytic core elements utilizing a corrugated thin metal foil honeycomb monoliths having a catalyst carried on or supported on their surface is the relatively expensive manufacturing cost of the monoliths. In addition, the leading edge of the monoliths must be reinforced to prevent flutter due to the impingement of the turbulent hot exhaust gases and to prevent crushing or collapse of the monoliths. Such reinforcement significantly increases the manufacturing costs of the catalytic converters and significantly reduces manufacturing speed.
A continuing concern in the catalytic converter industry, particularly in the automotive field, is the fact that excessive flow resistance or pressure loss reduces engine efficiency and performance. Thus, reducing pressure loss while maintaining effective emission control is a continuing industry goal. The term "pressure drop" as used herein means the difference between the pressure at the inlet face and the outlet face of the catalytic core element. The pressure drop across a catalytic core element of a catalytic converter generally used in the automotive industry is typically 2 inches of water at idle, and 12 to 15 inches of water at higher speeds. Higher pressure drops are undesirable, because the engine must expend extra energy to force the exhaust gas through the converter core element thereby reducing engine efficiency.
Accordingly, a need exists for a catalytic converter which is capable of withstanding the thermal stresses and vibrations encountered in typical catalytic converter installations and applications, is relatively easy and inexpensive to manufacture, has relatively long operating life, has a relatively low pressure drop, requires the same or less space than conventional catalytic converters, and can be manufactured having various cross-section configurations.